This invention relates to a magneto-optical recording medium and a reproducing method therefor. More particularly, the invention relates to a magneto-optical recording medium and a reproducing method therefor which can reproduce or read a fine recorded magnetic domain, much smaller than a reproducing light spot, in an enlarged form and which are suitable for high density recording.
A magneto-optical recording medium is capable of rewriting recorded information, has a large storage capacity, and is highly reliable as a recording medium. Thus, it begins to be put to practical use as a computer memory or the like. With increases in the amount of information and downsizing of equipment, however, a demand is growing for even higher density recording/reproducing technologies. To record information on a magneto-optical recording medium, a magnetic modulation method is used which irradiates the magneto-optical recording medium with constant laser light, and simultaneously applies a magnetic field of a polarity corresponding to a recording signal to a heated area of the medium. This method permits overwrite recording, and achieves high density recording, e.g., recording at the shortest mark length of 0.15 xcexcm. An optical modulation recording system for irradiating the medium with light power-modulated in response to recording signals under a constant magnetic field applied has also found practical use.
To reproduce or read record marks recorded at a high density, an optical reproduction resolution or resolving power determined by the spot size or diameter of a reproducing light beam should be given particular attention. For example, it is impossible to discern and reproduce a fine mark with a magnetic domain length of 0.15 xcexcm by the use of reproducing light with a spot diameter of 1 xcexcm. As one of the approaches for eliminating this restriction on reproduction resolving power by the optical spot diameter of reproducing light, a proposal has been made for magnetic: super-resolution (MSR) as described in the Journal of Magnetics Society of Japan, Vol. 17, Supplement No. S1, pp. 201 (1993). This technique utilizes the phenomenon that when reproducing light is cast on a magneto-optical recording medium, a temperature distribution occurs in a magnetic film within a reproducing light spot. Because of this phenomenon, a magnetic mask is generated in the spot to decrease the effective spot diameter contributing to the reproduction or reading of a signal. The use of this technology can improve reproduction resolving power without reducing the actual reproducing light spot diameter. However, this method makes the effective spot diameter small by the action of the magnetic mask. Thus, the amount of light contributing to a reproduction output decreases, and a reproduction C/N ratio declines accordingly. Consequently, it becomes difficult to obtain a sufficient C/N ratio.
Japanese Patent Application Laid-Open No. 1-143041 discloses a reproducing method for a magneto-optical recording medium, which uses a magneto-optical recording medium having a first magnetic film, a second magnetic film and a third magnetic film connected together magnetically at room temperature, the Curie temperatures of the first, second and third magnetic films being TC1, TC2 and TC3, respectively, with TC2 greater than room temperature and TC2 less than TC1, TC3, the coercivity of the first magnetic film, HC1, being sufficiently small at a temperature in the vicinity of the Curie temperature TC2 of the second magnetic film, and the coercivity of the third magnetic film, HC3, being sufficiently large compared with a required magnetic field in a temperature range from room temperature to a required temperature TPB higher than TC2,
whereby the recorded magnetic domains of the first magnetic film are enlarged to perform reproduction. This method utilizes a temperature rise of the medium upon irradiation with reproducing light to cut off the magnetic connection between the first and third magnetic films, and enlarges the magnetic domains of the first magnetic film by a demagnetizing field acting in the recorded magnetic domains in this condition and an externally applied magnetic field. With this technique, the second magnetic film used has a Curie temperature set at a lower value than the temperature of a readout area working during reproduction. The present invention, on the other hand, does not use a magnetic film with such magnetic characteristics.
It is an object of the present invention to solve the problems with the earlier technologies by a method different from the method described in Japanese Patent Application Laid-Open No. 1-143041, and provide a magneto-optical recording medium which gives reproduced signals at a sufficient C/N ratio even when fine magnetic domains are recorded, as well as a reproducing method for the signals.
According to a first aspect of the present invention, there is provided a magneto-optical recording medium having a magneto-optical recording film and an auxiliary magnetic film, and being adapted to reproduce a signal by magnetically transferring a recorded magnetic domain of the magneto-optical recording film to the auxiliary magnetic film upon irradiation with reproducing light, characterized in that:
the auxiliary magnetic film is a magnetic film of at least one layer which transforms from a plane-magnetized film into a perpendicular-magnetized film when exceeding its critical temperature, while the magneto-optical recording film is a perpendicular-magnetized film at a temperature not lower than room temperature;
whereby a larger magnetic domain than the recorded magnetic domain of the magneto-optical recording film can be transferred to the auxiliary magnetic film at the time of reproduction by virtue of the magnetic characteristics of the auxiliary magnetic film.
The magneto-optical recording medium of the present invention can be further classified into the following two types of magneto-optical recording media:
The first type of magneto-optical recording medium, as illustrated in FIGS. 2A and 2B, has a structure in which a first auxiliary magnetic film 5 and a second auxiliary magnetic film 4 are sequentially laminated on a magneto-optical recording film 6, the magneto-optical recording film 6, the first auxiliary magnetic film 5 and the second auxiliary magnetic film 4 having magnetic characteristics such that when the Curie temperatures of the magneto-optical recording film, the first auxiliary magnetic film and the second auxiliary magnetic film are designated as TC0, TC1 and TC2, respectively, and the critical temperatures of the first auxiliary magnetic film and the second auxiliary magnetic film are designated as TCR1 and TCR2, respectively, a relationship expressed as room temperature  less than TCR2 less than TCR1 less than TC0, TC1, TC2 is satisfied. The first auxiliary magnetic film 5 and the second auxiliary magnetic film 4, as shown in FIG. 3, have magnetic characteristics such that each of them acts as a plane-magnetized film in a temperature range from room temperature to a certain critical temperature (TCR) higher than room temperature, but acts as a perpendicular-magnetized film at a temperature above TCR. The magneto-optical recording film 6 is a perpendicular-magnetized film at a temperature not lower than room temperature.
The principle of action (reproduction) of the first type of magneto-optical recording medium will now be described.
FIG. 2A shows the state of magnetization of each layer before reproduction after writing recorded magnetic domains into the magneto-optical recording film 6 by the optical modulation recording system or the like. When this medium is irradiated with reproducing light of a suitable power for making the peak temperatures of the magnetic films desired temperatures, a magnetic domain 22 of perpendicular magnetization in the magneto-optical recording film 6 is transferred to an area in the first auxiliary magnetic film 5 where the temperature has become higher than TCR1. For this purpose, in view of a temperature profile within the medium developed upon irradiation with reproducing light as shown in FIG. 8, the reproducing power and TCR1 are set so that a magnetic domain 21 of the same size as, or a smaller size than, the size of the magnetic domain in the magneto-optical recording film 6 will be transferred to the first auxiliary magnetic film 5.
Then, the magnetic domain 21 transferred to the first auxiliary magnetic film 5 is transferred to the second auxiliary magnetic film 4. According to the present invention, the critical temperatures of the first and second auxiliary magnetic films are set to satisfy TCR2 less than TCR1. Thus, as indicated by the temperature profile within the medium of FIG. 8, an area in the second auxiliary magnetic film which can become perpendicularly magnetized is larger in diameter than that in the first auxiliary magnetic film. As shown in FIG. 2B, therefore, a transferred magnetic domain 23 in the second auxiliary magnetic film 4 is enlarged by perpendicular magnetic anisotropy within the perpendicularly magnetizable area in the second auxiliary magnetic film, and an exchange coupling force resulting from the perpendicular magnetization in the first auxiliary magnetic film 5. This magnetic domain enlargement can be said to be promoted, since the planar magnetization in areas indicated at W of the first auxiliary magnetic film 5 in FIG. 2B weakens the exchange coupling force from magnetic domains S of the magneto-optical recording film 6 into the second auxiliary magnetic film 4. This magnetic domain enlargement curtails the decrease in the quantity of light contributing to a reproduction output due to magnetic masking by plane magnetization, thus permitting reproduction at a high C/N ratio.
The effect of enlargement of the magnetic domain 23 in the second auxiliary magnetic film 4 becomes maximal when the transferred magnetic domain in the second auxiliary magnetic film 4 is enlarged to a size larger than the diameter of a reproducing light spot. In this state, a very large reproduction output is obtained which is unrelated to the size or shape of the magnetic domain recorded in the magneto-optical recording film 6 and which is determined only by the figure of merit of the second auxiliary magnetic film 4 and the intensity of reproducing light beam. After reproduction, namely, after the reproducing laser light-irradiated area has moved, the readout area is cooled to TCR2 or lower, whereupon the first and second auxiliary magnetic films are returned to a plane-magnetized state, the state of FIG. 2A. Even at temperatures during the reproducing action as described above, the coercivity of the magneto-optical recording film 6 is sufficiently high, so that the information recorded as magnetization is completely retained.
The second type of magneto-optical recording medium of the present invention, as illustrated in FIG. 7, is characterized by having a nonmagnetic film 9 between an auxiliary magnetic film 8 and a magneto-optical recording film 10, the magneto-optical recording film 10 and the auxiliary magnetic film 8 having magnetic characteristics such that when the Curie temperatures of the magneto-optical recording film and the auxiliary magnetic film are designated as TC0 and TC, respectively, and the critical temperature of the auxiliary magnetic film is designated as TCR, a relationship expressed as room temperature  less than TCR less than TC0, TC is satisfied.
The principle of reproduction of the second type of magneto-optical recording medium will now be described.
FIG. 6A schematically shows the state of magnetization of the auxiliary magnetic film 8, nonmagnetic film 9, and magneto-optical recording film 10 before reproduction after writing recorded magnetic domains into the magneto-optical recording film 10 of the medium shown in FIG. 7 by the optical modulation recording system or the like. When this magneto-optical recording medium is irradiated with reproducing light of a suitable power for making the peak temperatures of the magnetic films the desired temperatures, an area which can reach a temperature higher than TCR and can become perpendicularly magnetized occurs in the auxiliary magnetic film 8. The TCR and reproducing power are set so that the size of this area will become not smaller than the diameter of a magnetic domain M recorded in the magneto-optical recording film 10, preferably not smaller than the diameter of a reproducing light spot. The auxiliary magnetic film 8 has magnetic characteristics such that its coercivity has a distribution as shown in FIG. 9 in correspondence with a temperature distribution in the area above TCR, and the values of the coercivity are sufficiently small in a region reaching the peak temperature and in the vicinity of the region.
The magneto-optical recording film 10, on the other hand, has magnetic characteristics such that its magnetization has a distribution as shown in FIG. 9 in correspondence with the temperature distribution in the area above TCR, and the values of the magnetization are sufficiently large in a region reaching the peak temperature and in the vicinity of the region. Since the magnetic characteristics of the respective magnetic films have been set as described above, only the magnetic domain M in the high-temperature, sufficiently high magnetization region in the magneto-optical recording film 10 is transferred to the high-temperature, sufficiently low coercivity region in the auxiliary magnetic film 8 because of a great static magnetic coupling force between the magneto-optical recording film 10 and the auxiliary magnetic film 8 that acts in the region of the magnetic domain M. As a result, a sufficient reproduction resolving power is obtained.
Then, a magnetic domain 63 transferred to the auxiliary magnetic film 8 may have been enlarged by perpendicular magnetic anisotropy within the region above TCR, and exchange coupling force from the transferred magnetic domain as shown in FIG. 6B. This magnetic domain enlargement strengthens reproduced signals and increases a C/N ratio, as with the first type of magneto-optical recording medium. After reproduction, namely, after the reproducing laser light has moved, the readout area is cooled to TCR or lower, whereupon the auxiliary magnetic film 8 turns into a plane-magnetized film, and returns to the state of FIG. 6A.
According to a second aspect of the present invention, there is provided a reproducing method for a magneto-optical recording medium, which reproduces a recorded signal by irradiating the magneto-optical recording medium with reproducing light, and detecting the magnitude of a magneto-optical effect, the magneto-optical recording medium having a magneto-optical recording film which is a perpendicular-magnetized film at a temperature not lower than room temperature, the reproducing method being characterized by:
using as the magneto-optical recording medium a magneto-optical recording medium having a first auxiliary magnetic film and a second auxiliary magnetic film sequentially laminated on a magneto-optical recording film, the first and second auxiliary magnetic films being magnetic films which transform from plane-magnetized films into perpendicular-magnetized films when exceeding their critical temperatures, and the magneto-optical recording film, the first auxiliary magnetic film and the second auxiliary magnetic film having magnetic characteristics such that when the Curie temperatures of the magneto-optical recording film, the first auxiliary magnetic film and the second auxiliary magnetic film are designated as TC0, TC1 and TC2, respectively, and the critical temperatures of the first auxiliary magnetic film and the second auxiliary magnetic film are designated as TCR1 and TCR2, respectively, a relationship expressed as room temperature  less than TCR2 less than TCR1 less than TC0, TC1, TC2 is satisfied; and
irradiating the magneto-optical recording medium with reproducing light which is power-modulated with the same period as a reproduction clock or a period created by the multiplication of an integer (n) and the reproduction clock to reproduce a recorded signal.
According to a third aspect of the present invention, there is provided a reproducing method for a magneto-optical recording medium, which reproduces a recorded signal by irradiating the magneto-optical recording medium with reproducing light, and detecting the magnitude of a magneto-optical effect, the magneto-optical recording medium having a magneto-optical recording film which is a perpendicular-magnetized film at a temperature not lower than room temperature, the reproducing method being characterized by:
using as the magneto-optical recording medium a magneto-optical recording medium having an auxiliary magnetic film on the magneto-optical recording film via a nonmagnetic film, the auxiliary magnetic film transforming from a plane-magnetized film into a perpendicular-magnetized film when exceeding its critical temperature, the magneto-optical recording film and the auxiliary magnetic film having magnetic characteristics such that when the Curie temperatures of the magneto-optical recording film and the auxiliary magnetic film, are designated as TC0 and TC, respectively, and the critical temperature of the auxiliary magnetic film is designated as TCR, a relationship expressed as room temperature  less than TCR less than TC0, TC is satisfied; and
irradiating the magneto-optical recording medium with reproducing light which is power-modulated with the same period as a reproduction clock or a period created by the multiplication of an integer (n) and the reproduction clock to reproduce a recorded signal.
It is desirable that the reproducing light is power-modulated so as to have reproducing light powers Pr1 and Pr2 with the same period as the period of the reproduction clock or a period which the reproduction clock multiplied by an integer (nxe2x89xa72) makes, and one of the reproducing light powers Pr1 and Pr2 is a power for causing the magnetic domain enlargement of the auxiliary magnetic film.
The principle of the reproducing method complying with the third aspect of the present invention will be described with reference to FIG. 11, a schematic view of the reproducing method. In this reproducing method, the second type of magneto-optical recording medium shown in FIG. 6 is used. First, a predetermined record pattern as shown in FIG. 11(a) is recorded in the second type of magneto-optical recording medium by the optical modulation recording system or the like. In the drawing, record marks are recorded with the shortest mark pitch DP, and a record mark length DL is set such that DL=DP/2. During reproduction, pulsed laser light which has been modulated to two types of reproducing powers Pr2, Pr1 are projected onto the magneto-optical recording medium so that they will have a period DP synchronized with the record mark position and the emission width of the higher power Pr2 will be DL, as shown in FIG. 11(b). Light with the lower reproducing power Pr1 is projected always in an erase state (at sites without the record marks), while light with the higher reproducing power Pr2 is projected always in a record state (at sites with the record marks) and an erase state. FIG. 11(c) shows a reproduced signal waveform obtained by irradiation with reproducing pulsed laser light as shown in FIG. 11(b). FIG. 11(d), on the other hand, shows a reproduced signal waveform obtained by reproducing the same track with continuous light with a constant reproducing light power. Pr2 of Pr2 and Pr1 is selected to be a power which will bring about the magnetic domain enlargement of the auxiliary magnetic film 8, while Pr1 is selected to be a power which will cause the magnetic domain enlargement to vanish, as will be described later on. By so selecting the reproducing power, an amplitude Hp1 between the record state and the erase state observed with pulsed light reproduction can be set to have the relation Hp1 greater than Hdc, where Hdc is an amplitude with reproduction using constant laser light. Furthermore, magnetized information recorded in each magnetic domain of the magneto-optical recording film can be reproduced in an enlarged form independently of, and without influence from, adjacent magnetic domains.